Prevent ocean from behaving like a mirror
For large-scale water to look good, you'll need a few things:


An environment for the water to reflect. You already got this.
Waves. Water is almost never a perfectly flat mirror. You will need a lot of extra geometry to mimic the waves. It is a lot more common to use a displacement texture to 'fake' the effect.
A key property of water is fresnel reflection. That means depending on the angle of the surface is to the camera, it is either really dark or really reflective.


Combining all 3, this is what we end up:



The Blend file can be found here: https://www.dropbox.com/s/8pf88lwlobv3kf9/ocean.blend?dl=0